Administration of Amphotericin B (AmB) concommitant with platelet transfusion is associated with poor platelet recovery. This is of major importance when platelet transfusions and AmB are required simultaneously, as in patients who have acute leukemia or who have undergone bone marrow transplantation. We investigated in vitro the effect of AmB on platelet surface membrane glycoproteins (GP) and on platelet function to elucidate the platelet abnormality induced by AmB. Washed platelets exposed to AmB (4 microgram/ml) for 2h showed inhibition of aggregation by thrombin (0-0.6U/ml). The response of platelets to hypotonic stress was inhibited, mean platelet volume was decreased and percent microplatelets were decreased on treatment with AmB. Flow cytometric analysis, using fluorescently-labelled monoclonal antibodies (MAb) to the surface membrane GPs GPIb-IX, GPIIb-IIIa and P-selectin was carried out. Results showed that the activation-dependent antigen, P-selectin, was expressed on the surface of unactivated platelets exposed to AmB. Expression of GPIb-IX and GPIIb-IIIa in unactivated and thrombin-activated platelets was not affected by AmB. Platelet-PMN interactions were studied using MAb to GPIIb-IIIa (CD41) as a platelet marker and LDS-751, a nuclear stain, to label PMNs. Unactivated platelets treated with AmB showed an increase in the percent of platelets binding to PMNs and in the number of platelets bound per PMN, compared to control platelets. Exposure of platelets to AmB in vitro appears to induce abnormalities in platelet function, expression of P-selectin and platelet-PMNs interactions.